An estimation of an uncertainty caused by a direct propagation of the Level 1 ET sequence cut sets into the Level 2 risk

摘要

While the current risk-informed framework for a Level 2 PSA requires a complete transfer of all the Level 1 ET sequence information into the relevant PDSs (or Level 2 risk metrics), most practices on the PDS binning process often show an inconsistency between the Level 1 CDF and the Level 2 risk although both values must be theoretically the same in that case. Clarification of the foregoing issue is crucial since the PDS-related uncertainty analysis is a starting point of the Level 2 risk analysis and consequently whose portion is directly transferred to the Level 2 risk metrics. For this, a plant-specific impact of an analyst's different choice (or criteria) of the PDS definition has been analyzed with the way the Level 1 ET sequence cut sets are directly linked with the relevant PDSs during the Level 1/2 binning process. The present result shows that the foregoing difference is mainly caused by a combined impact of two competing uncertainty sources; (1) one source is a decrease of the PDS frequency due to a truncation limit and (2) the other source is its increase due to extra independent cut sets generated in the direct linking process of the Level 1 ET sequence cut sets with the relevant PDSs. From the numerical viewpoint, the plant-specific contribution of an uncertainty caused by the current Level 1/2 binning process to the Level 2 risk ranges from -1.2% to 2.5% of the Level 1 CDF value, consequently leading to the corresponding contribution to the Level 2 risk uncertainty.